1. Field of the Invention
The invention relates to a control apparatus and a control method for a hybrid vehicle. More particularly, the invention relates to a control apparatus and a control method that prevents gear noise caused by torque fluctuation in the engine from being produced when the drive torque of the vehicle is relatively small.
2. Description of the Related Art
Japanese Patent Application Publication No. 2005-161961 describes a type of hybrid vehicle in which either the front wheels or the rear wheels are driven by power from a first drive portion having an engine, first and second electric motors, and a gear type power split device that divides the power from the engine between the first electric motor and an output shaft, while the other wheels are driven by power from a second drive portion made up of a third electric motor. This kind of hybrid vehicle can run in several running modes, including a “motor two-wheel running” mode in which the engine is stopped and either the front wheels or the rear wheels are driven by power generated from the second electric motor, a “hybrid two-wheel running” mode in which either the front wheels or the rear wheels are driven by both the engine and the second electric motor, a “motor four-wheel running” mode in which the engine is stopped and either the front wheels or the rear wheels are driven by power generated from the second electric motor (i.e., as in the “motor two-wheel running” mode) and the other wheels are driven by power from the third electric motor as necessary, and a “hybrid four-wheel running” mode in which either the front wheels or the rear wheels are driven by both the engine and the second electric motor (i.e., as in the “hybrid two-wheel running” mode) and the other wheels are driven by power from the third electric motor as necessary.
In order to switch between the motor running mode and the hybrid running mode when the vehicle is moving, it may be necessary to start and stop the engine while the vehicle is in motion. However, a relatively large amount of torque fluctuation is generated when starting or stopping the engine so when the torque from the first drive portion is small, it temporarily becomes equal to or less than zero by the torque fluctuation in the engine, and as a result, gear noise may be produced from backlash in the gears in the gear type power split device which may result in a loss of comfort.
Also, in this kind of hybrid vehicle, the engine is typically operated at optimum fuel efficiency and torque control of the second electric motor is performed according to the required torque. However, because torque fluctuation increases if the engine speed is low, when the required torque is low such that the torque from the second electric motor is near zero, the torque from the first drive portion may temporarily become equal to or less than zero from torque fluctuation in the engine even if the engine is running. As a result, the gear noise may be produced from backlash of the gears in the gear type power split device as described above.